A stereotaxic arc system is a device commonly used in neurosurgery to quantitatively and accurately direct a probe into the body to a desired target seen on x-rays or tomographic imaging. There have been dozens of such devices invented in the last forty years. One class of such stereotaxic systems is the so-called target-centered arc concept. In this concept, an arc system which has been coupled by various means to the patient's body, as in the case of neurosurgery by fixation means to the skull, and the arc elements themselves can be rotated so as to provide radio guidance direction for a probe into the head. FIG. 1 illustrates the prior art in target-centered stereotaxic systems. Typically, the major arc element 1 is a so-called transverse arc which is usually a segment of a circular arc that rotates on a bearing or bearings 5a and 5b at one end, sometimes referred to as trunions. It further has a slide element 2 that runs along it which carries the probe guide 2a. With a combination of the rotation of the arc 1 about its axis and the probe slide 2 on the arc, one can achieve an infinite number of spherical radii 3 to the center 4 of the circular arc. The anatomical target, furthermore, is placed at the center of these arc radii, thereby enabling that all radii 3 will pass through that target 4. To move the target to the center of the arc, usually one has means to translate the arc in Cartesian or rectilinear movements, x, y and z, or in clinical jargon, anterior-posterior (A-P), lateral, and vertical. Typically, this is done by either moving the patient's head itself so that the target is at the center of the arc, or by moving the arc relative to the patient, who is secured in a head ring, to achieve the same end. Thus, the name target-centered arc means that the target is placed at the center of a spherical arc system. This construction has great utility'since once the target has been identified, that is its A-P, lateral, and vertical coordinates determined by x-ray or tomographic scanning, the target can be moved quantitatively by means of Cartesian slides so that the target lies at the center of the arc system. Thus, the surgeon may have great flexibility in choosing the desired arc settings for an optimal approach to the target.
There are many popular arc systems on the market today and designed over the past forty years which use this concept. Among them are the Todd-Wells Guide (USA), the Leksell System (Sweden), the Hitchcock (UK), the Laitinen System, and the Patil System (USA). All of these systems have the same basic characteristics. They have an arc system, which we can refer to as a transverse arc, which rotates about an axis relative to a base structure or base frame. The structure itself is coupled to the patient's head, and the head can be translated in Cartesian coordinates, x, y and z, or equivalently, A-P, lateral, and vertical, so that the anatomical target can be moved to what is the center of the radii that are defined by the circular arc. The circular transverse arc 1 can be moved on a bearing or trunion 5a or 5b about the axis of rotation 7. In addition, a probe guide carrier or transverse slide 2 moves along the circular arc to give a second angular degree of freedom. The combination of the arc rotation on its trunion or axis and the transverse slide movement can enable any probe direction 3 to be achieved. Since the target is at the isocenter of the two arc movements, that is all radii constitute radii for a sphere converging to the point 4 where the target 4 is located, then the probe automatically, when directed radially, will achieve the target from any of the slide or arc movement directions. This is the arc-centered or target-centered concept.
Although this target-centered design is simple and easy-to-use, in the two trunion designs prior to this invention there have been limitations in the angular range that the probe guide can achieve the target. In particular, the bearing or trunion 5a or 5b is always fixed in a particular location relative to the base frame 271 and thus relative to the patient's head, or to a body-fixed coordinate axis such as the anterior direction 6 as shown in FIG. 1. For example, in the Todd-Wells and Leksell, Patil, and Laitinen frames, the trunions are always located laterally or to the side of the patient's head (axis labeled by R (right) and L (left in FIG. 1), and thus at a fixed angular orientation relative to the target which is the center of the arc. This means that since the slide 2 cannot come down and pass the trunion position of 5a and 5b (or the bearing, if that is what supports the arc 1), then there is always a blind spot in the range of motion of the slide, where the probe cannot pass because it is obstructed by the trunion. Also, if you want a pure lateral approach, that is to say, an approach where the probe goes through the trunion hole, for example, you have limitations in the clearance between the probe and the trunion hole, which is restrictive in a surgical context. Typically too, these instruments have other structural obstructions to probe movement. For example, in the Leksell and Patil the transverse arc at certain angular orientations and slide positions will present an obstruction of the probe path by hitting the frame structure that supports the transverse arc and trunions itself.
The Hitchcock frame has only one trunion and only a partial arc segment for its transverse arc. This means it has a limited transverse arc range for a given trunion position and has less stability than the two trunion design show in FIG. 1. It does have a reorientable single trunion axis, but the limitations of the of the single trunion restrict its range and usefulness.
An objective of the present invention is to circumvent these limitations to the target-centered arc concept. The essential means by which we have done this in the present invention is to provide a way that the two trunions 5a and 5b themselves, and thus the axis of orientation 7 of their transverse arc, can be changed relative to the base frame 271 or the body-fixed coordinate axes such as 6 (anterior). For example, the trunions, instead of being placed laterally left and right on the patient's head, can be changed to an A-P, or anterior-posterior, orientation without having to release the patient's head from the frame and put the frame on at 90.degree.. This concept of a movable two trunion angular orientation relative to the patient's head has been described in a paper by Cosman and Wells presented at the Stereotaxic Society Meeting in Montreal, June 1987.
FIG. 1 is a diagram showing the prior art with regard to target-centered two trunion arc systems. The transverse arc 1 is attached to trunions 5a and 5b and can rotate about the axis 7 by bearings in the trunions. Transverse carrier probe guide 2a or direction-determining guide which guides the probe track 3 to the target 4. The trunion axis 7 is oriented from left (L) to right (R) as shown relative to the patient's head. The head is fixed to some other object in the apparatus not shown, such as a head ring, and that head ring can be translated in x, y, z Cartesian space relative to the arc target 4 so that the arc target position can be translated to any physiologic point in the patient's head. The salient point here is that in the prior art the two trunion axis 7 remains fixed in angular orientation relative to the patient's head once the head ring attachment to the arc system has been secured to the patient's head.
To describe the situation in FIG. 1 in another way, the rotation of transverse arc 1 around its trunions 5a and 5b constitutes one angular movement or degree of freedom of probe track 3. The probe slide 2 moving on transverse arc 1 constitutes a second and orthogonal angular degree of freedom. The combination of these two rotations constitutes a two-dimensional angular degree of freedom which means that probe direction 3 covers a range of directions within the limits of the movements of the slide 2 on arc 1 and the rotation of transverse arc 1 about its trunions 5a and 5b. All of the radial directions 3 established in this way pass through the center of the circular arc 1. This means that if an anatomical target is translated to the position 4 at the arc center, then all radii will pass through that target. Thus if probe carrier 2a can be used to guide a surgical instrument or a beam of particles, then that instrument or particles will pass through the anatomical target if it is placed at the arc center. The range of radii achievable in such a situation can be likened to a surface area on the unit's sphere and an accompanying solid angle of approach in the terminology of solid geometry. The ideal stereotaxic system would maximize the solid angle or range of two-dimensional angle of approaches as described above. It is an objective of this patent to describe an augmentation of this range of approaches which has to do with the reorientatability of axis 7.